The only well known and active alternative implementation of JSF is not just an alternative, but a very competitive offering.
In the JSF 1.x days, this used to be the only viable solution (the reference implementation, RI, was terrible and bug ridden back then). With 2.x the situation has improved for the RI (now called Mojarra), but MyFaces still holds its own in a couple of areas.
For instance, dynamic component tree manipulation just doesn't work correctly on Mojarra. On MyFaces it works flawless.
Performance is another big topic. In some versions Myfaces is faster, in some other versions Mojarra. There doesn't seem to be a definite winner here.
An important difference between MyFaces and Mojarra is that MyFaces by default serializes the so-called view state when that is kept on the server. This can become a serious performance bottleneck, but there's an option to switch it off. Incidentally, Mojarra has an option to turn it on.